Using TCP Through Sockets

client s
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client s
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child
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finger
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Figure 1: file descriptors in a forking server are inherited by the child process (top
figure). In the second figure, the child process has called dup2 to connect file descriptors
1 and 2 which normally correspond to standard output and standard error to the network
socket. In the final figure, the child has called execl and the local finger process runs, sending
its standard output over s.
Accept takes a socket s on which one is listening and returns a new socket to which
a client has just connected. If no clients have connected, accept will block until one
does. accept returns −1 on an error.
For TCP, addr should be a struct sockaddr in *. addrlenp must be a pointer
to an integer containing the value sizeof (struct sockaddr in). accept will adjust
*addrlenp to contain the actual length of the struct sockaddr it copies into
*addr. In the case of TCP, all struct sockaddr in’s are the same size, so *addrlenp
shouldn’t change.
The finger daemon makes use of a few more Unix system calls which, while not networkspecific,
are often encountered in network servers. With fork it creates a new process. This
new process calls dup2 to redirect its standard output and error over the accepted socket.
Finally, a call to execl replaces the new process with an instance of the finger program.
Finger inherits its standard output and error, so these go straight back over the network to
the client.
• int fork (void);
fork creates a new process, identical to the current one. fork returns twice: in the old
process, fork returns a process ID of the new process. In the new or “child” process,
fork returns 0. fork returns −1 if there is an error.
• int dup2(int oldfd, int newfd);
dup2 closes file descriptor number newfd, and replaces it with a copy of oldfd. When
the second argument is 1, this changes the destination of the standard output. When
that argument is 2, it changes the standard error.
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